Problems with repositories and aptitude update

I am installing (or trying to!) Trinity on a fresh minimal install of Squeeze. But I have run into problems (which I did not the other day on a full install). I have edited my sources.list as instructed on the Pearson website, and have checked and rechecked the repositories. They seem to be fine, but when I do <aptitude update> I get a lot of error messages like this one: [ ERR] Reading package lists E: Encountered a section with no Package: header E: Problem with MergeList /var/lib/apt/lists/ppa.quickbuild.pearsoncomputing.net_trinity_trinity_debian_dists_squeeze_main_i18n_Translation-en E: The package lists or status file could not be parsed or opened. E: Couldn't rebuild package cache And obviously couldn't install Trinity either, with similar errors. I am completely baffled. Can anyone throw light on this? Lisi